1. Technological Field
The present invention relates to a wheel slip control system for a vehicle, operable so as to control the rotation of the driven wheels to control the power of internal combustion engine, as well as control the braking system for driven wheels during an acceleration slip.
2. Prior Art
There have been known various kinds of anti-skid control systems (for example, Published unexamined patent application sho No. 59-2962 "Hydraulic Anti-skid Control System") for vehicles which control the slip of the wheels during braking and acceleration operation. On the other hand, another slip control system for vehicles which conducts traction control for improving running stability, acceleration, etc. has been considered. In such a device, idle running of driven wheels is prevented and rotation of driven wheels is controlled so as to maximize the frictional force between the tire and road surface during acceleration operation.
As for the latter traction control system, it is considered that the control of internal combustion engine power and that of rotation of driven wheels are carried out by adjusting, e.g., the ignition timing and the fuel injection amount when the degree of driven wheel slip is found to exceed a predetermined value.
However, the wheel slip control systems mentioned above are still inert, and the following problems remained unsolved.
The conventional wheel slip control systems aim at the prevention of slip during acceleration by reducing the fuel injection amount or setting a time lag against a change in the ignition timing so as to control the output of the internal combustion engine. In this case, there is a problem that the control capacity is limited, due to the necessity of taking preventive measures against abnormal vibration or fire due to abrupt changes in the operating conditions of the engine.
On the other hand, when the intake air amount is controlled, the ignition timing and the fuel injection amount, etc. are determined in accordance with it. Therefore, the output of internal combustion engine can be controlled smoothly to improve the drivability. But if the opening position of the throttle valve interlocked to the accelerator pedal, is controlled against the commanded position of the pedal commanded by being stepped on, some problems may occur. For example, discomfort like kickback to the driver, or incapability of keeping stability in case of failure with control parts of the throttle valve may exist.
In addition, another problem, that is, the rotation of driven wheels cannot be controlled immediately because of the bad responsiveness, should be pointed out when the output of internal combustion engine is used for controlling the rotation of driven wheels in the case above. Accordingly, it is possible to control the rotation of driven wheels directly by using the brake system mounted on the vehicle, when the slip of driven wheels occurs during acceleration operation and the rotation of them must be controlled instantly. But a special brake system will be required to control the rotation of driven wheels by using the brake system only. Namely, if the driving force is great, for example, when the gear position of transmission is set at the first speed, the special brake system which produces greater brake force will be necessary to give the corresponding brake force against the driving force. The conventional brake system may not work well. Also, the pressure source will be required for the purpose of increasing braking pressure supplied to the braking cylinder equipped with the driven wheels of the control system for vehicles. Therefore, an oil pump must be added to the conventional vehicles in order to activate the brake for throttle control (A small-type pump is acceptable).